The drive train of a wheeled excavator typically includes a diesel engine that drives, among other things, a variable displacement hydraulic piston pump. This pump provides hydraulic fluid under pressure required to drive one or more hydraulic motors, that in turn drive some or all of the wheels of the excavator, typically via a reduction gearbox. The pump may also supply hydraulic motors and/or hydraulic cylinders provided to control other components of the excavator, such as the boom, the arm, the bucket, or the like.
In one known hydraulic drive system for a wheeled excavator, the wheels of the excavator are driven by a single hydraulic motor through a constant mesh, non-synchro mesh, multi-speed gearbox. Although this arrangement is very efficient, it suffers from the drawback that excavators employing this system are not particularly operator friendly. In particular, because the gearbox is a non-synchromesh box, it is necessary to bring the excavator to a complete standstill before changing gear.
A second known hydraulic drive system for a wheeled excavator addresses the above problem by using a "power-shift", constant mesh, multi-speed gearbox, through which the wheels of the excavator are driven by a single hydraulic motor. This system is easier to use than a system with a non-synchro box, the "power-shift" gearbox enabling gear changes to be made while the vehicle is moving. However, the gearbox used in this second system is necessarily more complex than that of the first system, resulting in a more expensive and less efficient hydraulic drive.
A third known hydraulic drive system for a wheeled excavator provides for further ease of use whilst avoiding the expense and complexity of the "power-shift" gearbox. This third system incorporates two drive motors both of which are variable displacement motors and one of which is capable of zero minimum displacement.
More particularly, this system includes one high speed, low torque hydraulic motor and one low speed, high torque hydraulic motor, the low speed motor being capable of being set to zero displacement. Both the motors are variable displacement hydraulic motors and are engaged with the input side of a single ratio, reduction gearbox, via which, in combination, they drive wheels of the excavator. The "gear change" is accomplished by altering the displacement of the motors, and thus requires minimal input from the operator. This arrangement is therefore easier to use than either of the first or the second above mentioned hydraulic drive arrangements.
However, while being easy to use, this third arrangement is more expensive than the first described arrangement. Although the gearbox is simple, it is necessary to use two motors. Additionally, and most significantly, the efficiency of this third arrangement is lower than that of the first and second arrangements.
In order to overcome this problem and to improve the efficiency of the system, it has been proposed to introduce a clutch between the low speed motor and the gearbox arranged to disengage said motor from the drive train during travel once the low speed motor becomes redundant to drive of the vehicle.
However, control of this clutch, if operated on the move, has heretofore been achieved using a sophisticated control system to operate the clutch at precisely the correct moment--i.e. when the displacement of the low speed motor is zero, and when the rotational speed thereof is less than a predetermined value. It has so far proved difficult to provide a suitable clutch control system of commercial viability.